In power application circuits, semiconductor devices are typically utilized to convert, apply, and remove electrical power. Conventional semiconductor devices are capable of switching (e.g., turning on and off) at high speeds (e.g., high frequency) and are capable of linearly controlling the application of electrical power to a load. Semiconductor devices can include diodes, insulated gate bipolar transistors (IGBTs), thyristors, silicon-controlled rectifiers (SCRs), field effect transistors (FETs), bipolar transistors, semiconductor switches, and combinations thereof. Power application circuits can perform a variety of operations, such as, controlling motors and generators, generating power, transmitting power, converting power, discharging and charging capacitors, dissipating power, disconnecting power, connecting power, or high voltage and high current functions.
Unlike conventional electromechanical switches (e.g., electromagnetic contactors, relays, and reed switches), the semiconductor devices can advantageously switch, or turn on or off, at high speeds (e.g., over several kilohertz, KHz). However, semiconductor devices generally have a power loss greater than conventional electromechanical switches. The power loss occurs when the device is conducting current (e.g., is in a conduction state) and when the device is switched (e.g., during switching to or from the conduction state). The power loss, P.sub.loss, is typically dissipated as heat and can be represented by the following equation: EQU P.sub.loss =IV.sub.d +I.sup.2 R.sub.on +KV.sub.s I.sub.s F
where: I=the current through the semiconductor device when the device is in a conduction state; PA1 Rm=the conduction resistance of the semiconductor device; PA1 V.sub.d =the ON state voltage drop associated with the device when it is in the conduction state; PA1 V.sub.s =the voltage across the semiconductor device during switching; PA1 I.sub.s =the current through the semiconductor device during switching; PA1 K=a constant related to characteristics of the device; and PA1 F=the frequency at which the semiconductor device is switched.
The term KV.sub.s I.sub.s F represents the switching losses, and the sum of the terms I.sup.2 R.sub.on and IV.sub.d represents the conduction losses. Thus, in power applications where the terms I, V.sub.s, and F can be quite large, a considerable amount of electrical power is consumed when the semiconductor device is switching and conducting.
The power loss associated with semiconductor devices can be as large as 5% of the rated power output of the power application circuit. For example, in conventional motor drives, IGBTs transform input voltages into switched output voltages, which are applied to a motor to create desired torque and speed. The IGBTs can have a power loss of about 5% of the rated power of the motor, depending upon the size of the motor. This large power loss is a particularly important design constraint because the drive must be designed to limit the temperature rise in the devices and in the motor drive. Silicon junctions associated with the semiconductor devices, such as, IGBTs, cannot have a temperature which exceeds approximately 125.degree. C. Therefore, the design of motor drives must include significant cooling apparatus, such as, large heat sinks and blowers, to maintain the semiconductor devices at a moderate temperature. Heat sinks and blowers add considerably to the size, weight, and cost of the motor drive. Thus, large power losses due to the semiconductor devices must be accommodated in conventional power application circuits.
Semiconductor devices can be designed either to reduce switching losses at the expense of increasing conduction losses or to reduce conduction losses at the expense of increasing switching losses. For example, switching losses in IGBTs can be reduced by lowering the lifetime of charge carriers in the channel region. The lifetime can be lowered by a number of processes, such as, irradiating the IGBT with an electron beam or doping epitaxial layers of the IGBT with metallic substances (e.g., gold or platinum). However, reducing the lifetime of the charge carriers tends to increase the conduction losses associated with the IGBT. Despite the optimization of semiconductor devices to have conduction losses and switching losses for particular power application criteria, power application circuits still have high power losses due to the use of semiconductor devices.
Conventional electromagnetic switches which have low conduction losses cannot be utilized in many power application circuits because the electromagnetic switches are too slow. For instance, conventional electromagnetic switches cannot be utilized in AC to AC, DC to DC, AC to DC, and DC to AC power conversion circuits because these circuits often require that the switch be turned on and off thousands of times per second (e.g., in the kilohertz (KHz) range). Additionally, power application circuits often require fast actuation speeds (e.g., that the switch be opened or closed in a small amount of time).
Some conventional electromagnetic switches have been utilized in parallel with IGBTs in AC and DC motor drives to directly connect the power source to the motor, once the motor has reached (e.g., ramped up to) an operating speed. However, the IGBTs still are responsible for high power losses as the motor is ramped up, slowed down, or otherwise controlled by the IGBTs. Further, this scheme cannot be utilized in power inverter applications.
Some conventional relay switches have been included in an inverter bypass kit. The relay switch shorts out the inverter to reduce power loss. Other conventional relay switches have been used to short out FETs on high power digital output cards. However, these relay switches cannot operate at high frequencies. Thus, conventional electromagnetic switches or relay switches do not solve the power loss problem associated with semiconductor devices in power application circuits.
Additionally, conventional electromechanical switches can be inappropriate for use in power conversion and application circuits because of the many cycles of operation which they must perform during the life of power conversion and application circuits. For example, during the 20-year life of a power conversion circuit, the switch in a power application circuit can perform over 6 trillion cycles. The performance of electromagnetic switches can degrade and wear over the lifetime due to the mechanical nature of the switches. Further, in high power applications, conventional electromagnetic switches can be prone to arcing, which provides additional wear (e.g. contact erosion) on the mechanical switches which in many cases increases or generates resistance across the switch contacts.
Thus, there is a need for a power conversion circuit which has reduced power loss. Further still, there is a need for electromechanical switches which are capable of switching at high frequencies. Even further still, there is a need for a power conversion circuit which can utilize electromechanical switches. Yet further, there is a need for an integrated circuit package which includes an apparatus for reducing power loss in a semiconductor device.